The present invention relates to a coding technology for use in transmitting picture signals.
Picture signals, especially, television signals which are typical motion picture signals, represent high correlation between frames i.e. high interframe correlation. By making use of this high interframe correlation, it is possible to reduce i.e. compress a large volume of information at the time of transmission. A system called "interframe predictive coding" is the simplest system among the methods of utilizing interframe correlation. See H. Kaneko and T. Ishiguro "Digital television transmission using bandwidth compression techniques", IEEE Communication Magazine, July 1980, pp. 14-22. With this system, the smaller the number of moving portions included in a frame, the greater the compression ratio becomes. However, when the extent of motion (area, or velocity etc.) is large, a large degree of compression cannot be expected. On the other hand, a "Motion-compensated interframe predictive coding system" has been proposed to always realize adaptively an optimum prediction with respect to moving portions, even when they are included, by utilizing a motion vector indicative of moving speed and direction of the moving portions. This motion-compensated interframe coding method makes it possible to realize a compression to a great extent even if moving portions are included in the frame. For instance, an example of the motion-compensated interframe coding system is described in U.S. Pat. No. 4,460,923 assigned to the same assignee as the present invention. This patent discloses a predictive coding system capable of remarkably improving coding efficiency especially where the transmission speed is low. In a code conversion which uses as small a quantity of codes as possible for expressing a prediction error or a motion vector obtained as a result of high efficiency predictive coding, and coding parameters or horizontal and vertical synchronizing signal etc. used in the predictive coding, a variable length code e.g. Huffman code etc. is often used.
Alternation of the coding parameter is not necessarily carried out every scanning line. Ordinarily, such alternation is effected with N scanning lines (N.gtoreq.1), e.g., N=8 as a unit. Accordingly, it is convenient to effect coding conversion per N scanning lines. FIG. 1 schematically illustrates an example of a simple code conversion having a code sequence labelled B. As seen from this figure, the coding procedure comprises providing in succession a synchronizing code (e.g. zero of 8 bits) indicating that a vertical or horizontal synchronizing signal subsequently arrives, a line synchronizing code representative of horizontal synchronization of N scanning lines, a coding mode signal and a motion vector code signal, and coding a prediction error obtained when a predictive coding is effected by using the motion vector code signal and its coding mode signal, thus completing code conversion in regard to N scanning lines. The synchronizing code, the line synchronizing code and the coding mode signal have bit configuration of 8 bits, repsectively, and 24 bits are used in total for expressing these signals Assuming that a picture signal system has M scanning lines, 24.times.M/N bits per one frame are required. For instance, when M=525 and N=8, 1575 bits per one frame are required. In a system where 30 frames per second are indicated, the bit rate amounts up to 1575.times.30.div.47 Kbps (bit per second), which corresponds to the fixed quantity of information ordinarily produced. Namely, even in the case where both the quantity of the motion vector and that of the prediction error represent negligible values nearly equal to zero, the rate at which information occurs is 47 Kpbs. Assuming now that the bit rate of a communication channel used in the transmission is 1.5 Mbps, the ratio of 47/1500.div.1/30 is not so high. However, in the case where the bit rate of the communication channel lower than 1.5 Mbps, e.g., 500 Kbps is assumed, the ratio becomes 47/500.div.1/10 which is considerably large in the occupation ratio. Accordingly, when transmitting a picture signal at a low transmission speed or bit rate, in the case where a motion vector and a prediction error are repeated in the same manner every N scanning lines under the same coding mode, a method which enables code conversion with a reduced quantity of codes is required.